1. Field of the Invention
This invention relates to thermoplastic resin compositions having good heat resistance or a combination of good heat resistance and impact resistance. More particualrly, it relates to resin compositions which comprise a polyvinyl chloride resin and a mixture of a copolymer (A) containing .alpha.-methylstyrene as a main component and a graft copolymer (B) obtained by graft polymerization of a conjugated diene rubber with an aromatic vinyl compound and a unsaturated nitrile compound or an alkyl methacrylate, and it relates to thermoplastic resin compositions having greatly improved resistance to heat distortion and impact resistance over those of polyvinyl chloride resins.
2. Description of the Prior Art
Polyvinyl chloride resins have excellent mechanical characteristics and electrical characteristics and possess characteristic physical properties such as transparency, flame retardency etc. as well as versatile working characteristics, and thus they are extensively employed as useful plastic materials in various fields of applications. However, the resins of this type have thermal distortion temperature of 65.degree.-70.degree. C., and therefore their use is limited where the articles are subjected to high temperature environments, for example, pipes for hot water, automobile interior parts etc. where the indoor temperature may become very high and the like.
Heretofore, as methods to improve this disadvantage of the polyvinyl chloride resins, there have been various methods provided, e.g., those involving incorporating an AS (acrylonitrile - styrene) resin, an ABS (acrylonitrile - butadiene - styrene) resin and an MBS (methyl methacrylate - butadiene - styrene) resin, respectively, to polyvinyl chloride resins, and there has also been proposed a method which comprises introducing .alpha.-methylstyrene monomer as one component of these resins in order to further enhance the effect to improve heat resistance. For example, Japanese Patent Publication No. 24867/1970 describes that a composition having high heat resistance is obtained by blending a polyvinyl chloride resin with a copolymer of methyl methacrylate, acrylonitrile, styrene and .alpha.-methylstyrene and a butadiene containing polymer. However, the amount of .alpha.-methylstyrene in the copolymer is merely 45% by weight at best and heat resistance is not adequate. Further, Japanese Patent Publication No. 18101/1973 describes a method which comprises incorporating a resin (A) obtained by copolymerizing a monomer mixture consisting of 5-50% of methyl methacrylate, 3-30% of acrylonitrile and 30-80% of .alpha.-methylstyrene together with a butadiene containing graft polymer into a polyvinyl chloride resin. However, by this method, it can be estimated from the reactivity on copolymerization that it is practically impossible to make the .alpha.-methylstyrene content in this copolymer (A) 70% by weight or higher even locally.
We have discovered that high heat resistance which has not been achieved by the above-described prior art techniques can be obtained by incorporating a copolymer (A) which is a copolymer comprising 65-90 parts by weight of .alpha.-methylstyrene with 35-10 parts by weight of acrylonitrile and 0-5 parts by weight of a copolymerizable vinyl monomer or monomers and containing 30 parts by weight or more of a component having an .alpha.-methylstyrene content of 82% by weight or higher, and a graft copolymer (B) obtained by reacting 35-80% by weight of a conjugated diene rubber with 65-20% by weight of a monomer comprising a monovinyl aromatic compound and a unsaturated nitrile compound or an alkyl methacrylate into a polyvinyl chloride resin, and thus accomplished this invention.